A FOLDER DIPOLE ANTENNA

Abstract: A folded dipole antenna (100) includes a dielectric substrate (120) for defining a first mounting surface (142) and a second mounting surface (144) substantially parallel thereto. A folded dipole element (154) is mounted on the second mounting surface. The antenna further includes a microstrip feed line (150), mounted on the first surface. In one implementation of the antenna the microstrip feed line is connected to a coaxial cable (140). In this implementation the folded dipole element includes a continuous dipole arm (162) arranged parallel to first and second dipole arm segments (166, 170) separated by an excitation gap. The feed line (150) is mounted in alignment with the excitation gap and is electrically connected to the continuous dipole arm (162). The antenna may additionally include a ground plane (110) separated from the folded dipole element by a dielectric spacer (130) for altering, in a predetermined direction, the radiation pattern of the folded dipole element. The thickness of the dielectric spacer between the ground plane reflector and the folded dipole element is selected to determine the impedance presented by the antenna to the coaxial cable.

Description: A FOLDED DIPOLE ANTENNA The present invention relates to the field of antennas, and particularly but not exclusively to a folded dipole antenna. BACKGROUND OF THE INVENTION In the design of portable radio equipment, and in particular personal paging devices, size is an extremely important factor. Many previous paging devices employed relatively large receive antennas, thereby significantly increasing overall device dimensions. Antennas of this scale were generally required as a consequence of the use of relatively low RF paging frequencies, and also so as to ensure adequate reception of the paging signals. Specifically, high antenna gain is desirable, and under certain conditions may in fact be necessary to ensure achievement of full receiver range capability. However, size constraints preclude incorporation of conventional high gain antenna configurations into paging receivers designed to be relatively compact. The large size of many conventional paging receivers has required that they be mounted on the side of the body, usually through attachment to the belt or through placement in a pocket. Recently, however, it has been desired to realise paging devices sufficiently compact to be, for example, worn on the wrist. One advantage offered by wrist-carried paging receivers is that they may be held in front of the face, thereby facilitating viewing or adjustment by the user. Existing wrist-carried paging receivers often include simple loop type antennas responsive to the magnetic field component of the RF signal. In such antennas the loop element is generally disposed within the wrist band of the user. Although this type of antenna system has tended to provide only marginal performance, it enables the loop antenna to be concealed within the wrist band housing. However, this arrangement is of advantage only if it is desired that the attachment mechanism consist of a wrist band or other loop-type device. Accordingly, it would be desirable to provide an antenna system which is capable of being implemented within a paging receiver of compact dimension, and which does not presuppose a particular type of attachment mechanism. As noted above, receive antennas incorporated within conventional terrestrial paging devices have tended to be somewhat large, partially as a consequence of the use of relatively low paging frequencies (e.g., < 1 GHz). However, existing satellite communications systems operative at, for example 1.5 GHz (receive)/l.6 GHz (transmit) or 2.5 GHz, afford the opportunity for paging receiver antennas of smaller scale. Antennas operative at these frequencies would need to have gains sufficiently low to project broad radiation patterns, thus enabling reception of paging signals from a broad range of angles. This is required since terrestrial reception of satellite signals is based not only upon line-of-sight transmissions, but also upon transmissions scattered and reflected by objects such as buildings, roads, and the like. Hence, it would be desirable to provide a compact antenna capable of receiving paging signals from communication satellites. It will be appreciated by those possessed of the appropriate skills that for a given antenna the forms of the transmission gain and reception sensitivity characteristics are the same. As used herein, therefore, the term "radiation pattern" means both the transmission gain and the reception sensitivity characteristics of an antenna. SUMMARY OF THE INVENTION The invention aims to provide a folded dipole antenna which mitigates at least some of the above- discussed problems. According to one aspect of the invention there is provided a folded dipole antenna comprising: a dielectric substrate defining a first surface and a second surface substantially parallel to said first surface; a folded dipole element on said second surface; and a feed element on said first surface. According to another aspect of the invention there is provided a folded dipole antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface; a folded dipole element mounted on said second mounting surface; a microstrip feed line mounted on said first surface in alignment with at least a portion of said folded dipole element; and directing means for directing the radiation pattern of said folded dipole element away therefrom. According to a further aspect of the invention there is provided a folded dipole antenna for a paging receiver, the antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface; a folded dipole element mounted on said second mounting surface, said folded dipole including a continuous arm and first and second dipole arm segments arranged to be substantially parallel to said continuous arm and having a characteristic radiation pattern; a microstrip feed line mounted on said first surface in alignment with an excitation gap defined by ends of said first and second folded dipole arm segments; reflecting means for affecting the radiation pattern of said folded dipole element; and means for supplying a received signal from said microstrip feed line to a receiver. The invention also provides a paging receiver comprising a housing for housing receiver circuitry, a folded dipole antenna attached to a first external surface of said housing, and an auxiliary antenna mounted on a second external surface of said housing, the folded dipole antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface, a folded dipole element mounted on said second mounting surface, said folded dipole including a continuous arm and first and second dipole arm segments arranged to be substantially parallel to said continuous arm and having a known radiation pattern, a microstrip feed line mounted on said first surface in alignment with an excitation gap defined by ends of said first and second folded dipole arm segments, a ground plane for increasing the radiation pattern of said folded dipole element in directions away from the housing, and means for supplying a received signal from said microstrip feed line to said receiver circuitry. The invention further provides an antenna in which a feeder is formed on one side of a laminate and a folded dipole is formed on the other side of the laminate, the folded dipole being spaced from a ground plane by a dielectric spacer. The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure l shows a personal paging receiver incorporating a folded dipole antenna; Figure 2 provides an illustration of the microstrip structure of the folded dipole antenna of Figure 1; Figure 3 depicts an implementation of the folded dipole antenna in greater detail, providing a cross- sectional view from which the housing has been omitted for clarity; Figure 4 shows a partially see-through top view of a folded dipole antenna embodying the invention; Figure 5a provides a scaled representation of a folded dipole microstrip circuit element; Figure 5b provides a scaled representation of a feeder line microstrip circuit element; and Figure 6 is a graph showing the driving point resistance at the centre of a horizontal 1/2 wavelength antenna as a function of the height thereof above a ground plane. DETAILED DESCRIPTION OF THE DRAWINGS Referring to Figure l, there is illustrated a personal paging receiver in which is incorporated a folded dipole antenna system embodying the invention. The paging receiver designated generally as 10 includes a display 20 and input switches 30 for operating the paging receiver in a manner well known to those of ordinary skill in the art. The receiver 10 is disposed within a housing 40, a lateral side of which provides a surface for mounting an auxiliary microstrip patch antenna 50. In addition, the housing 40 defines a first end surface on which is mounted a folded dipole antenna 100. As is indicated by Figure 1, the auxiliary patch antenna 50 is designed to project a radiation pattern having an electric field orientation El transverse to the electric field orientation E2 of the dipole antenna 100. This combination of antennas facilitates improved reception of paging signals of diverse polarization and angle of incidence. In an exemplary implementation (to be described in greater detail hereinafter) the folded dipole antenna 100 is designed to receive paging signals broadcast via satellite at a frequency of 1542 MHz. As shown in Figures l and 2, the inventive folded dipole antenna 100 is implemented using a microstrip structure comprising an antenna ground plane 110, a microstrip laminate board 120, and a foam spacer 130 interposed therebetween. The antenna 100 will generally be attached to the housing 40 by gluing the ground plane 110 thereto using, for example, a hot- melt plastic adhesive. The ground plane 110 may be δ fabricated from a metallic sheet having a thickness within the range of 0.5 to 2.0 mm, and includes an external segment 110a for connection to a lateral side of the housing 40. The foam spacer 130 may be fabricated from, for example, polystyrene foam having a dielectric constant of approximately 1.2. The thickness of the foam spacer 130 is selected in accordance with the desired impedance, typically 50 ohms, to be presented by the antenna 100 to a coaxial cable 140 (Figure 2) . Referring to Figure 2, the cable 140 extends from receiver electronics (not shown) into the foam spacer 130 through a slot defined by the ground plane 110. As is described below, the inner and outer conductors of the coaxial cable 140 are connected, using a conventional coaxial-to-microstrip transition, to printed microstrip circuit elements disposed on the upper and lower surfaces 142 and 144, respectively, of the laminate board 120. The microstrip laminate board may for example comprise a Duroid sheet, typically of a thickness between 1 and 2 mm, produced by the Rogers Corporation of Chandler, Arizona, or any other similar PTFE based microwave printed circuit board laminate. Microstrip substrates composed of other laminate materials, e.g. alumina, may alternatively be utilized. Figure 3 illustrates the folded dipole antenna 100 in greater detail, providing a cross-sectional view from which the housing 40 has been omitted for clarity. As shown in Figure 3, a feeder line 150 comprising microstrip circuit elements is printed on the upper surface 142 of the microstrip laminate board 120. In addition, a folded microstrip dipole element 154 is printed on the lower surface 144 of the board 120. In an exemplary embodiment the centre conductor of the coaxial cable 140 extends through the laminate board 120 into electrical contact with the feeder line 150. Similarly, the outer conductor of the coaxial cable 140 makes electrical contact with the folded dipole 154 through the outer collar of a coaxial-to- microstrip transition 158. Referring to Figure 4, there is shown a partially see-through top view of the folded dipole antenna 100. As shown in Figure 4, the folded dipole microstrip element generally indicated by the dashed outline 154 includes a continuous arm 162 , as well as first and second arm segments 166 and 170. The first and second arm segments 166 and 170 define an excitation gap G which is spanned from above by the feeder line 150. In use the folded dipole 154 excites the feeder line 150 across the excitation gap G, which results in an excitation signal being provided to receive electronics (not shown) of the paging receiver via the inner conductor 178 of the coaxial cable 140. In this regard the folded dipole 154 provides a ground plane for the feeder line 150, and is in direct electrical contact therewith through a wire connection 180 extending through the microstrip board 120. The ground plane 110 (Figure 3) operates as an antenna reflector to vary the radiation pattern projected by the folded dipole 154. Specifically, ground plane 110 redirects the radiation pattern in directions away from the receiver housing 40. Although in the embodiment of Figure 1 it is desired to maximise the radiation pattern in directions away from the receiver housing 40, in other applications it may be desired that the folded dipole antenna produce beam patterns in both vertical directions relative to the folded dipole 154. Accordingly, it is expected that in such other applications that the dipole antenna would be implemented absent a ground plane element. The folded dipole 154 and feeder line 150 microstrip circuit elements can for example be realised using a laminate board having a pair of copper-plated surfaces. Each surface is etched in order to produce copper profiles corresponding to the folded dipole and feeder line elements. Alternatively, these elements could be realised by directly plating both sides of a laminate board with, for example, gold or copper, so as to form the appropriate microstrip circuit patterns. Figure 5a and 5b provide scaled representations of the folded dipole 154 and feeder line 150 microstrip circuit elements, respectively. In the representation of Figures 5a and 5b the dimensions of the feeder line and dipole have been selected assuming an operational frequency of 1542 MHz and a laminate board dielectric constant of approximately 2.3. The dimensions corresponding to length (L) , width (W) , and diameter (D) parameters of the microstrip elements represented in Figure 5 are set forth in the following table. It is noted that parameter D3 refers to the diameter of the circular aperture defined by the laminate board 20 through which extends the centre conductor of coaxial cable 140. Similarly, the parameter D2 corresponds to the diameter of a circular region of the continuous dipole arm 162 from which copper plating has been removed proximate the aperture specified by D3. This plating removal prevents an electrical short circuit from being developed between the centre coaxial conductor and the folded dipole 154. Advantageously, an end portion of the centre coaxial conductor is soldered to the microstrip feeder line 150 after being threaded through the laminate board 120 and the dipole arm 162. The overall size of the dipole antenna may be adjusted to conform to the dimensions of the paging receiver housing through appropriate dielectric material selection. For example, the microstrip circuit dimensions given in TABLE 1 assume an implementation using Duroid laminate board having a dielectric constant of approximately 2.3. A smaller folded dipole antenna could be realised by using a laminate board consisting of, for example, a thin alumina substrate. Referring again to Figure 3, it is observed that the separation between the folded dipole 154 and the ground plane 110 is determined by the thickness T of the foam spacer 130. The thickness T and dielectric constant of the foam spacer 130 are selected based on the desired impedance to be presented by the folded dipole antenna (for example the impedance of the folded dipole antenna should be matched to the 50 ohm impedance of the coaxial cable 140) . As is described below, one technique for determining the appropriate thickness T of the foam spacer 130 contemplates estimating the driving point impedance of the folded dipole antenna. Such an estimate may be made using, for example, a graphical representation of antenna impedance such as that depicted in Figure 6. In particular, Figure 6 is a graph of the impedance of a conventional 1/2 wavelength dipole antenna situated horizontally above a reflecting plane, as a function of the free-space wavelength separation therebetween. As is indicated by Figure 6, the impedance for large separation distances is approximately 73 ohms, and is less than 73 ohms if the dipole is situated close to (e.g., less than 0.2 wavelengths) and parallel with a reflecting plane. A folded 1/2 wavelength dipole exhibits an impedance approximately four times greater than the impedance of a conventional 1/2 wavelength dipole separated an identical distance from a reflecting plane. Accordingly, the separation required to achieve an impedance of 50 ohms using a folded dipole is equivalent to that necessary to attain an impedance of 12.5 ohms using a conventional 1/2 wavelength dipole. In order to use Figure 6 in estimation of the impedance of a folded dipole separated from a reflecting plane by a dielectric spacer the free-space separation distance must be further reduced by the factor 1/Je, where e denotes the dielectric constant of the spacer. Thus, in accordance with Figure 6, the separation required to achieve an impedance of 50 ohms for a folded 1/2 wavelength dipole, using a dielectric space with a dielectric constant of approximately 1.2 would be approximately (lfVl .2 ) x 0.075 wavelengths, or approximately 0.07 wavelengths. Thus, a relatively thin dielectric spacer can be used. It will be appreciated by those possessed of the appropriate skills that whilst the antenna has been described for use with a receiver it is equally suitable for use with an antenna since the radiation pattern and other characteristics are the same for both transmit and receive. While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.

Claims: CLAIMS : 1. A folded dipole antenna comprising: a dielectric substrate defining a first surface and a second surface substantially parallel to said first surface; a folded dipole element on said second surface; and a feed element on said first surface. 2. The antenna of claim 1, wherein said feed element comprises a microstrip feed line. 3. The antenna of claim l or 2, wherein said folded dipole element includes a continuous dipole arm arranged parallel to first and second dipole arm segments separated by an excitation gap ' , said feed element being mounted in alignment with said excitation gap and electrically connected to said continuous dipole arm. 4. The antenna of claim 1 or 2 or 3, further including reflector means for altering the radiation pattern of said folded dipole element. 5. The antenna of claim 4, wherein the reflector means comprises a ground plane, the dipole element being positioned between the feed element and the ground plane and being spaced from the ground plane by way of a dielectric spacer. 6. A folded dipole antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface; a folded dipole element mounted on said second mounting surface; a microstrip feed line mounted on said first surface in alignment with at least a portion of said folded dipole element; and directing means for directing the radiation pattern of said folded dipole element away therefrom. 7. The antenna of claim 6, wherein said directing means includes a ground plane positioned a predetermined distance from said second mounting surface. 8. The antenna of claim 7, wherein said reflector means further includes a dielectric spacer interposed between said ground plane and said second mounting surface. 9. The antenna of claim 8, further comprising a coaxial cable extending through said dielectric spacer, said cable having a centre conductor electrically connected to said microstrip feed line and an outer conductor electrically connected to said dipole element. 10. The antenna of claim 8 or 9, wherein thickness of said dielectric substrate is selected such that the impedance presented by said antenna to said coaxial cable is approximately fifty ohms. 11. The antenna of any of claims 6 to 10, wherein said folded dipole element includes a continuous dipole arm arranged parallel to first and second dipole arm segments separated by an excitation gap. 12. The antenna of claim 11, wherein said microstrip feed line is mounted in alignment with said excitation gap and is electrically connected to said dipole arm. 13. A folded dipole antenna for a paging receiver, the antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface; a folded dipole element mounted on said second mounting surface, said folded dipole including a continuous arm and first and second dipole arm segments arranged to be substantially parallel to said continuous arm and having a characteristic radiation pattern; a microstrip feed line mounted on said first surface in alignment with an excitation gap defined by ends of said first and second folded dipole arm segments; reflecting means for affecting the radiation pattern of said folded dipole element; and means for supplying a received signal from said microstrip feed line to a receiver. 14. The microstrip antenna of claim 13, wherein said reflecting means includes a ground plane positioned a predetermined distance from said second mounting surface. 15. The microstrip antenna of claim 14, wherein said reflecting means further includes a dielectric substrate spacer interposed between said ground plane and said second mounting surface. 16. The microstrip antenna of any of claims 13 to 15, wherein said means for supplying a received signal includes a coaxial cable extending through said dielectric spacer and said ground plane for connection to a paging receiver, said cable having a centre conductor electrically connected to said feed line and an outer conductor electrically connected to said dipole element. 17. A paging receiver comprising a housing for housing receiver circuitry, a folded dipole antenna attached to a first external surface of said housing, and an auxiliary antenna mounted on a second external surface of said housing, the folded dipole antenna comprising: a dielectric substrate for defining a first mounting surface and a second mounting surface substantially parallel to said first mounting surface, a folded dipole element mounted on said second mounting surface, said folded dipole including a continuous arm and first and second dipole arm segments arranged to be substantially parallel to said continuous arm and having a known radiation pattern, a microstrip feed line mounted on said first surface in alignment with an excitation gap defined by ends of said first and second folded dipole arm segments, a ground plane for increasing the radiation pattern of said folded dipole element in directions away from the housing, and means for supplying a received signal from said microstrip feed line to said receiver circuitry. 18. The antenna of claim 17, including a dielectric spacer interposed between said ground plane and said second mounting surface, wherein the thickness and/or dielectric constant of said dielectric spacer is selected such that the impedance presented by said antenna is approximately fifty ohms. 19. An antenna in which a feeder is formed on one side of a laminate and a folded dipole is formed on the other side of the laminate, the folded dipole being spaced from a ground plane by a dielectric spacer. 20. An antenna or a receiver incorporating an antenna substantially as described herein with reference to the accompanying drawings.

Inventors: KEEN KEITH MALCOLM, KEEN, KEITH, MALCOLM
Applicants: INT MARITIME SATELLITE ORGANIZ, INTERNATIONAL MARITIME SATELLITE ORGANIZATION
Application year: 1994
Main classification: H01Q1
IPC codes: H01Q1, H01Q9